Boosting long-haul microwave capacity with 1024 QAM

Long-haul microwave systems in telecommunication networks carry information across distances of more than 15 km and even more than 200 km in a single hop. They are deployed in a wide variety of applications such as mobile networks, carrier networks, offshore links, digital broadcasting and many more. The alternative, fiber cable, is often not available due to cost, deployment time, regulations, reliability, security, terrain difficulties or other limiting factors. Easy and fast-to-deploy, microwave is often the medium of choice for long-haul applications.

Microwave systems use digital modulation techniques to carry information in free space over an analog carrier signal. As use of mobile telephones, laptops and other computer devices increases along with demand for data-rich services like video, network operators are bombarded with requirements to increase capacity in the transport network. As microwave spectrum is limited and is often encumbered with expensive frequency fees, advances in modulation techniques can contribute significantly to microwave spectrum efficiency and thus to capacity improvements.

ModulationA modulation technique uses some number of distinct signals, symbols, to represent the digital information that is to be transmitted. Modulation techniques are considered to be more efficient when each symbol can represent a greater quantity of information—more bits. Modulation techniques are important considerations in the efficiency and capacity of a microwave system.An analog carrier, like a microwave signal over the air, can be modulated to carry digital information in three ways: by amplitude, by frequency and/or by phase. Each modulation technique varies a parameter of the signal to represent the information that is being transmitted. Microwave links and other types of wireless transmission equipment employ various modulation techniques to boost the amount of information that can be transmitted in a given period of time, viz. capacity.

Each modulation technique has advantages and disadvantages. For example, amplitude modulation, in a common implementation called amplitude shift keying (ASK), is affected by noise and interference. Frequency shift keying (FSK) behaves better than ASK in the presence of interference, but is more expensive to implement. Phase shift keying (PSK) is a better performer than either ASK or FSK and is used frequently in digital communications.

Amplitude, Frequency and Phase Modulation Techniques

PSK also gives rise to a family of modulation techniques starting with binary-PSK, or BPSK. In BPSK, a single bit—a zero or one—is sent per symbol (change of phase). More advanced transmission systems make use of quadrature PSK, 8PSK, 16PSK, etc. In each case, PSK is able to increase the capacity of the signal by coding for more bits per symbol. For example, Quadrature PSK (QPSK) is effectively two independent BPSK modulations and provides twice the capacity. Quadrature Amplitude Modulation (QAM)QAM is a very advanced modulation technique and is today’s state of the art in microwave radio transmissions. In QAM, the transmitter splits the flow of bits (information) to be transmitted into two equal parts modulating two independent waves. A 90-degree phase is created between the waves. QAM represents information by modulating the amplitude of the two waves that are out of phase with each other to represent the signal. In effect, QAM is amplitude modulation and phase modulation combined.

Amplitude and Phase Modulation Combined

The advent of QAM has given rise to a series of increasingly efficient modulations. It is easy to understand the efficiencies. In QAM, a symbol can represent two or more bits of information. As with all modulation techniques, the greater the number of bits of information per symbol, the more capacious the transmission.

5000iP SerHello Erik
Good to hear that you have used 1024 QAM over a 65 km path in Western Norway, using the 6.7 GHz frequency band.
We are also looking to deploy Higher capacity Microwave Link for long distance above 65 Km using higher order modulation like 1024 QAM.
Could you please share me some technical parametrs ( like TX power, antenna type, antenna dimension, antenna gain, Channel Bandwidth, Modulation,Rx level,Rx sensitivity, Fade margin,Channels) with the link budget calculation used. Which Company Microwave radio you are using for such link.
Hope you information would be very helpful to me and my companyies

Hi, indeed 1024QAM pose huge challenges in several areas. The peak to average ratio depend strongly on how the 1024QAM constellation is implemented. In the radio described in this case the peak to average for 1024QAM is really not much different to 256QAM. It does require Tx linearization and sufficient power backoff, but compared to 256QAM it’s really not very different. Long Haul microwave radios operate typically with 28-30, 40 or 56 MHz carrier bandwidth (varying with different frequency bands). With such bandwidth, it is indeed susceptible to multipath fading. The debate of OFDM vs single carrier, is not clear cut. OFDM offers both pros and cons in this area. OFDM is potentially offering better robustness to selective (multipath) fading but requires more signal processing and secondly the OFDM signal could offer better spectrum utilisation (due to the more “square” spectrum) but spectrum masks issued by the regulatory bodies (FCC, ETSI) are optimised to single carrier modulation.

1024 QAM presents a huge challenge to both tx and rx. On the ttx side, 1024 QAM has a much higher peak to average power ratio which means a big issue to the non-linear power amp. Would that be a compromise between the linearisation technique and the power backoff? On the RX side, it is susceptible to multiopath fading, which can imply a very expensive adaptive equaliser to compensate it. Given the maturity of technologies such as OFDM or SC FDE, should the system be considered with one of these advanced technologies?

Thanks for the interesting article. I'd like to know more about the additional complexity and cost needed to increase the throughput. It seems like you will definitely need more linear components, lower phase noise and maybe more DSP. Does the extra cost scale with the extra throughput? Or does it cost more than 1.25x to get 1.25x data throughput?

I guess it's important to understand that we are talking about carrier grade long haul (transport) links here. These links are dimensioned/engineered for very high availability, typically in the range of 99.995 or 99.999 % availability. This means less than half hour of outage per year! In order to achieve these figures the links have ample fading margin; typically in the range of 30 to 40 dB. Increasing modulation to 1024QAM reduces fading margin with 6dB compared to 256QAM. The effect on availability is just a factor of about 2 (doubling the outage), considering flat fading and interference free conditions. It has very limited effect on distance, as these links are not operating at the maximum possible distance anyway (due to needed fading margin). Interference can be an issue, again the sensitivity is increased by 6dB, so it needs to be factored into the availability calculations. Higher lever modulation does not change power consumption or other factors. So bottom line; as it uses adaptive modulation it offers 25% more capacity for 99.99% of the time. For the 0.01% of the time the links will scale down to lower modulation.
We are running a trial link over a 65 km path in Western Norway, using the 6.7 GHz frequency band. This has been running so far for about 2 months without even once changing to lower modulation. I hope this has helped clarify the technology.

It seems to me a waste of hardware upgrade. Performance requirements such as SNR for 1024-QAM is so high that it will appear to be defensless against channel fading, not to mention interferers. With adaptive modulation, it turns out to be running as 256-QAM at best as before. But it can be a good fit to wired channels such as cable, etc.